Cancer of the uterine cervix or cervical cancer is the second most common cancer in women worldwide. Although screening programs to identify precursor lesions of cervical cancer have contributed to a reduction in mortality and morbidity, 500,000 new cases of invasive cervical cancer are diagnosed worldwide and 230,000 women die of cervical cancer annually. Early detection and diagnosis is critical for survival of cervical cancer.
Infection by human papillomaviruses (HPV) at specific epithelium cells to induce epithelial proliferations plays an important role for cervical carcinogenesis. About 99 percent of confirmed cervical cancer cases are found to be associated with HPV infection with biopsy-confirmed squamous intraepithelial lesions (SIL) or cervical intraepithelial neoplasia (CIN). The incidence of HPV infection, primarily transmitted through sexual contact, is highest among young women and about 20 millions of sexually active men and women worldwide are currently infected. Approximately 1% of the population has genital warts and 4% of women have cervical precancerous lesions, such as low grade of squamous intraepithelial lesion (LGSIL) or high grade of squamous intraepithelial lesion (HSIL). The presence of these lesions, preferentially observed in women aged 35-40 yrs, are at high risk of progression toward invasive cancer.
There are more than one hundred genetic types of human papillomaviruses identified so far and only a relative few types of HPV, such as HPV-16, -18, -31, -33, -35, -45, -51, -52 and -56, etc., involve high risk of progression from HPV infected genital tissue lesions to invasive cervical cancer. Infection with the vast majority of HPV types, such as HPV-6 and -11, etc., are transient with no permanent changes in genital tissues and are at low risk for developing into invasive cervical cancer. However, the development of cervical cancer is a multiple step process that cannot be explained simply by infection with specific types of HPV. Persistent infections with HPVs in high risk group are essential but not exclusively required for the initiation of cervical carcinogenesis. It is found that younger age group women are often infected with HPV; however, clinical information reveals that most latent or asymptomatic infections with high risk HPV types as well as early dysplastic lesions (CIN 1) are usually self-limited and regress spontaneously. There is a high level of correlation between long term persistent infections with only few high-risk HPV types and the induction of advanced CIN 2/CIN 3 lesions and/or the progression to invasive cancer.
One additional event that appears to play a role in tumor progression is the integration of HPV DNA genome into host genome, which frequently disrupts the open reading frame for an early viral gene, E2, resulting in over-expression of two important viral E6 and E7 oncoproteins and transformation of the host cells. Since almost all cervical cancer cases harbor high risk-HPV genomes, screening with HPV infection is important, especially long term infection with high risk HPV types. Other factors and mutational or secondary genetic events may also be important in the progression and pathogenesis of invasive cervical cancers, including recombination, integration of viral genes to host cell chromosomes, chromosomal rearrangements, loss of constitutional heterozygosity, and proto-oncogene activation.
In the past, screening for cervical cancer is based on conventional cytology by papanicolaou (Pap) smear and suspicious smears are followed up with colposcopy, and/or histological biopsy. The use of cytological screening lead to a remarkable reduction in the mortality of cervical cancer. However, due to subjective test criteria, drawbacks of pap smear tests include difficulty in obtaining samples, poor inter- and intra-observer agreement, a high rate of false negatives (up to 20%) and false positive, the requirements for specialized labs staffed with highly trained personnel, and inability to identify a large proportion of HPV-infected persons. More reproducible assays are needed to improve the current screening method and avoid unnecessary medical intervention and psychological distress for the affected women. Nucleic acid methods, such as “DNA Hybrid Capture”, have been developed, but are not ideal primarily due to their high cost, assay operation procedures, and the requirements for facility, equipment, and highly trained personnel. What is needed is a low cost, simple, sensitive and specific assay that can be performed on routine practice of a clinical lab or doctor office.
Attempts to detect the presence of HPV related antibodies in a human subject by ELISA (enzyme linked immunoabsorbant assays) generally lead to extremely low assay sensitivity and thus can not be developed into a commercially suitable diagnostic test. Most of these ELISA assays target a single viral protein or short peptide fragments, which were not able to interact well or bind strongly and specifically to antibodies from the human subject. The assay specificity and sensitivity are so low such that even using samples from patients confirmed with HPV associated invasive cervical cancer, only 53% of the patient samples were found positive for HPV infection. Thus, there is no successful ELISA assay available as a diagnostic tool for clinical samples.
Therefore, there is a need to develop methods and assays for early detection of HPV infection and assisting in diagnosis of cervical cancer.